1. Filed of Invention
The present invention relates to power converters, and more particularly, to the control of power converters.
2. Description of Related Art
The switching controller is an integrated circuit used in the power converter to control and regulate the switching duty-cycle. Being subject to environmental regulations, the power system design of computers and other electrical products have been required to meet the power management and energy conservation standards. For the power management application in the power converter, how to reduce the power loss and acoustic noise in light-load and no-load condition is a main concern.
FIG. 1 shows a schematic diagram of a power converter. The power converter comprises a transformer 10 having a primary winding NP and a secondary winding NS. The primary winding NP is coupled to an input voltage VIN. The secondary winding NS generates an output voltage VO via an output rectifier 40 and an output capacitor 45. The output rectifier 40 is coupled to a first terminal of the secondary winding NS. The output capacitor 45 is coupled to a second terminal of the secondary winding NS and the output rectifier 40. In order to regulate the output voltage VO, a switching controller 100 generates a switching signal SOUT at an output terminal OUT to switch the transformer 10 via a power switch 20. The power switch 20 is coupled to the primary winding NP. When the power switch 20 is turned on, the input voltage VIN is applied to magnetize the transformer 10. A switching current IS therefore flows through the primary winding NP of the transformer 10 and the power switch 20. Through a current-sense device 30, the switching current IS is converted to a current signal VI coupled to a current sense terminal VS of the switching controller 100. The current-sense device 30 is coupled between the power switch 20 and a ground.
A feedback voltage VFB coupled to a feedback terminal FB of the switching controller 100 is derived from an output of an optical-coupler 60. An input of the optical-coupler 60 is connected to the output voltage VO of the power converter through a resistor 51 and a zener diode 50 to form a voltage feedback loop. Once the switching signal SOUT is disabled and the power switch 20 is turned off, the demagnetizing of the transformer 10 is started. The energy stored into the transformer 10 is delivered to the secondary winding NS during the demagnetizing of the transformer 10. Decreasing the switching frequency or stopping (burst) the switching of the power converter will reduce the power loss at the light-load of power converter. However, the transformer 10 of the power converter might generate acoustic noise if the switching frequency of the power converter falls into the audio band. Following equation shows a magnetic force F generated by the switching current IS in the transformer 10.F=B×l×N×IS where B: flux density; l: conductor length; N: winding-turns.Refer to the above equation, we can find limiting the switching current IS can limit the magnetic force F and reduce the acoustic noise.